The first electronic instrument amplifiers were designed for use with electric guitars. The earliest examples appeared in the early 1930s. Tone controls on early guitar amplifiers were very simple and provided a great deal of treble boost but the controls the loudspeakers used and the low power of the amplifiers had poor high treble and bass response. Guitar amplifiers are manufactured in two main forms. The "combination" (or combo) amplifier contains the amplifier head and guitar speakers in a single unit. The amplifier head or "amp head" contains the electronic circuitry constituting the preamp, built-in effects processing, and power amplifier. In the other form, the amplifier head is separate from the speakers, and joined to them by cables. A separate amplifier head placed atop a guitar speaker enclosure or guitar speaker cabinet forms an amplifier stack or "amp stack".